The present invention generally relates to ink jet recording methods and apparatuses, and more particularly to an ink jet recording method which enables gradation recording and an ink jet recording apparatus which employs such an ink jet recording method.
Recently, there is growing interest in non-impact recording methods because noise generated at the time of the recording is negligibly small according to these methods. Among such non-impact recording methods, the so-called ink jet recording method is an effective method because a high-speed recording is possible and the recording can be made on an ordinary paper without the need for a special fixing process. Various kinds of ink jet recording methods have been proposed in the past, and some have already been reduced to practice while others are still being modified.
The ink jet recording methods eject droplets of ink and adhere the droplets onto a recording medium such as paper. The ink jet recording methods can be categorized into several systems depending on the methods of generating the droplets of ink and the methods of controlling the ejecting direction of the droplets.
A first method is disclosed in a U.S. Pat. No. 3,060,429, for example. According to this first method, the droplets of ink are generated by electrostatic suction and the droplets are controlled by an electric field depending on a recording signal so that the droplets are selectively adhered on the recording medium. More particularly, the electric field is applied between a nozzle and an accelerating electrode, and the nozzle ejects uniformly charged droplets of ink. These droplets are ejected between x-y deflection electrodes which are electrically controlled depending on the recording signal, and the droplets are selectively adhered on the recording medium depending on the intensity change of the electric field.
A second method is disclosed in U.S. Pat. Nos. 3,596,275 and 3,298,030, for example. According to this second method, charge-controlled droplets of ink are generated by a continuous vibration generating method, and the droplets are ejected between deflection electrodes applied with a uniform electric field and adhered on the recording medium. More particularly, a recording head having a piezo vibration element and a nozzle is employed, and a charging electrode applied with a recording signal is arranged in front of an orifice of the nozzle at a predetermined distance from the orifice. An electric signal having a constant frequency is applied to the piezo vibration element so as to mechanically vibrate the piezo vibration element, and the droplets of ink are ejected via the orifice. The droplets which are ejected are charged by the charging electrode due to electrostatic induction, and the droplets are charged by an amount dependent on the recording signal. The charge-controlled droplets are deflected depending on the amount of charge as they are ejected between deflection electrodes which apply a uniform electric field, and only the droplets which carry the recording signal are adhered on the recording medium.
A third method is disclosed in a U.S. Pat. No. 3,416,153, for example. According to this third method, an electric field is applied between a nozzle and a ring-shaped charging electrode, and the droplets of ink are generated in the form of mist by the continuous vibration generating method. In other words, according to this third method, the mist state of the droplets is controlled by modulating the field intensity applied between the nozzle and the charging electrode depending on the recording signal, and the recording is made on the recording medium with gradation.
A fourth method is disclosed in a U.S. Pat. No. 3,747,120, for example. The operating principle of this fourth method differs completely from those of the first, second and third methods described above. In other words, the first through third methods electrically control the droplets of ink ejected from the nozzle, and the droplets carrying the recording signal are selectively adhered on the recording medium. But according to the fourth method, the droplets of ink are ejected from the nozzle depending on the recording signal. That is, the electric recording signal is applied to the piezo vibration element of the recording head which has the nozzle so as to convert the electric recording signal into the mechanical vibration of the piezo vibration element, and the droplets of ink are ejected from the nozzle depending on this mechanical vibration so as to adhere the droplets on the recording medium.
However, each of the four methods described above have problems to be solved, as will be described hereinafter.
According to the first through third methods, the droplets of ink are generated directly from electrical energy, and the deflection control of the droplets is made by the electric field. For this reason, the first method uses a simple construction, but a large voltage is required to generate small droplets of ink. In addition, the first method is unsuited for a high-speed recording because it is difficult to provide a multi-nozzle on the recording head.
As for the second method, the high-speed recording is possible because the multi-nozzle may be provided on the recording head. However, the construction needed to generate the droplets of ink becomes complex, and it is difficult to electrically control the small droplets. Furthermore, the so-called satellite dots are easily formed on the recording medium.
The third method can record a satisfactory image with gradation by forming a mist of the droplets of ink. But in this case, it is difficult to control the mist state, and smear is easily formed on the recording medium. Furthermore, it is difficult to provide the multi-nozzle on the recording head, and the third method is unsuited for carrying out the high-speed recording.
Compared to the first through third methods, the fourth method has a relatively large number of advantageous points. In other words, the fourth method uses a simple construction. In addition, since the droplets of ink are ejected from the nozzle in an on-demand manner, it is unnecessary to recover the droplets which are not used for the recording, unlike the first through third methods. Moreover, unlike the first and second methods, the fourth method does not require the use of a conductive ink, and the material and composition of the ink can be selected with a large degree of freedom. But on the other hand, it is difficult to form the recording head required by the fourth method. Furthermore, it is difficult to provide the multi-nozzle on the recording head because the downsizing of the piezo vibration element having a desired resonance frequency is extremely difficult. The fourth method is also unsuited for carrying out the high-speed recording because the droplets of ink are ejected by the mechanical energy, that is, the mechanical vibration of the piezo vibration element.
Therefore, there is a problem in that the first through fourth methods can only be used in applications where the disadvantages of each method can substantially be neglected.
An ink jet recording apparatus was previously proposed in a Japanese Laid-Open Patent Application No. 54-51837 to reduce the problems described above. According to this proposed ink jet recording apparatus, the ink within an ink chamber is heated so as to generate air bubbles and the pressure of the ink is increased. As a result, the ink is ejected from a fine capillary tube nozzle and transferred onto a recording medium such as paper. Using the operating principle of this proposed ink jet recording apparatus, various modifications have been made.
A Japanese Laid-Open Patent Application No. 2-23349 proposes one of such modifications. According to this modification, a thermal ink jet recording head ejects droplets of ink from a capillary tube region in response to an electrical signal. The thermal ink jet recording head is provided with a head resistor which generates heat in response to the electrical signal and is provided at a position such that the heat is applied within the capillary tube region. The head resistor includes a resistor region and a conductor region which surrounds the resistor region. At least a part of the conductor region is electrically connected to the resistor region. By providing the conductor region at the central part of the resistor region, the air bubbles are generated in a ring shape when a current pulse is applied, and a cool point is generated at the center of the head resistor. As a result, the air bubbles are destroyed into smaller air bubbles and the smaller air bubbles are distributed at random on the surface of the head resistor due to the shock of this destruction, thereby making it possible to minimize the damage caused by the cavitation of the head resistor.
The durability of the ink jet recording apparatus proposed in the Japanese Laid-Open Patent Application No. 54-51837 has also been improved recently and reduced to practice. However, the demand to more finely control the amount of ink ejection is increasing so that it is possible to obtain an even finer image quality. A Japanese Laid-Open Patent Application No. 55-132259 proposes one method of satisfying such a demand.
According to the method proposed in the Japanese Laid-Open Patent Application No. 55-132259, a sudden state transition is caused in the ink by the action of the thermal energy, and droplets of ink are ejected by the action based on this state transition so as to adhere the ink on the recording medium. More particularly, an ejection orifice for ejecting the ink in a predetermined direction is provided at a terminal end of a conduit, and a thermal action part is arranged to communicate with the ejection orifice and to effectively transmit the action force generated therein in the direction of the ejection orifice. This thermal action part is formed by at least two electric-to-heat converters capable of independently receiving signals, and a gradation recording is carried out by appropriately shifting the timings of the signals input to the electric-to-heat converters. As may be understood from the teaching in the Japanese Laid-Open Patent Application No. 55-132259, a control electrode and a ground (common) electrode are formed on the same plane. For this reason, there is a problem in that it is difficult to arrange the electrodes with a high density.
On the other hand, Japanese Laid-Open Patent Applications No. 55-73568 and No. 55-73569 propose selecting a predetermined number of heaters out of a plurality of heaters arranged in one conduit or, selecting one heater from a plurality of heaters having mutually different heat values, so as to vary the size of the air bubbles which are generated and to control the amount of ink which is ejected. However, the air bubbles generated in the thermal ink jet recording apparatus displays a binary behavior ("1" or "0"), that is, the ink is either ejected or not ejected. Therefore, the amount of ink which is ejected inevitably changes in steps, and a smooth change cannot be realized. For this reason, there is a problem in that it is difficult to make the recording with a high image quality.